Cities are now home to half of the world's 6.6 billion humans. By 2030, nearly 5 billion people will live in cities. This special issue explores the enormous implications of the mass embrace of city life. News articles offer a look at how cities are tackling specific problems, a set of Reviews and Perspectives examines trends and demographics arising from the urban transformation.

As Grimm et al. (p. 756) show, cities are hot spots of production, consumption, and waste generation. Already, according to the United Nations, cities are responsible for 75% of global energy consumption and 80% of greenhouse gas emissions. Without careful investment and planning, megacities (those with more than 10 million inhabitants) will be overwhelmed with burgeoning slums and environmental problems. There are advantages to city life, such as the relative proximity of health care (Dye, p. 766) and jobs. However, Mace (p. 764) describes continuing costs in terms of fertility, and Bloom et al. (p. 772) challenge a commonly accepted perception that urbanization fuels economic growth.

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Watch a video introduction on the problems, prospects, and science of cities, featuring researchers Michael Batty, Nancy Grimm, and Jesse Ausubel, as well as Science Asia News Editor Richard Stone.

Cities have taken novel approaches to dealing with urbanization. A News article (p. 740) explores how the Chinese government is encouraging a variety of schemes, including the development of “eco-cities.” Other News items highlight success stories, including Bogotá's reduction of traffic fatalities (p. 742), London's reduction of traffic jams (p. 750), and Mexico's efforts to alleviate urban poverty (p. 754).

The pace of urbanization is accelerating throughout the developing world (Montgomery, p. 761). One of the most pressing issues for these cities is the provision of clean water and sanitation. News articles feature three cities with different solutions: Durban (p. 744), Salvador, (p. 745), and Phnom Penh (p. 746). Sometimes, however, a lack of money and powerful lobbies can thwart the best intentions, as people in Kolkata have learned as they try to clean up their city's foul air (p. 749).

How will cities evolve? Batty (p. 769) shows that in spite of the apparently amorphous growth of urban sprawl, resilient patterns emerge. He advocates the use of complex systems analysis in future urban planning. Preparing for natural disasters, and recovering from them, will also challenge planners—especially because many of the world's largest cities lie on coasts and are vulnerable to flooding as the climate warms (p. 748).

Someday, cities may grow their own crops and raise their own livestock in vertical farms (p. 752). Next-generation hybrid cars could help cut greenhouse gas emissions (p. 750). A more distant dream is a “supercity” that relies on superconducting electricity cables and liquid hydrogen for its energy needs (p. 753). Futuristic concepts, perhaps, but the time has come for a radical rethink of our concept of cities and their place in the global environment.